DSP-128 upgrading?

jumunius · Posted: Mon Dec 13, 2010 11:10 am Post subject: DSP-128 upgrading?

I popped open a dusty old Digitech DSP-128 effects processor to change a battery and give it another listen after 10 years. I remember it being noisy, and I guess that's its reputation. Since most of the ICs are socketed I was thinking of upgrading some of the opamps to see if I can improve it. But there aren't any schematics online, and nobody talks about modding this thing (maybe people figure Quadraverbs of the same vintage are more worthy of their time? or maybe the AD/DA converters are the problem?).

Anyways, I was thinking of maybe replacing some LF353's with a couple OPA275/OPA2134 I have lying around, but there are so many of them. Anyone have any advice on where I might start swapping? Chips nearest the pots, the ins, the outs, etc.? I want to keep the 80's digital verb character, just clean up the signal.

Here are all the ICs I recognize as opamps:
(2) 5532 by ins and dry outs
(13) LF353
(2) LF357
(1) LM324 near input/headroom LEDs

Any suggestions would be greatly appreciated.

jumunius · Posted: Tue Dec 14, 2010 11:13 am Post subject:

Well, in case anyone is keeping track, I tried swapping LF353s near the jacks for some OP275 I had lying around, and noticed a small improvement. But the thing still sounded pretty noisy, the pots were all in need of cleaning or replacing, and it was powering really inconsistently. Doesn't seem worth all the work (when I could cheaply get an old Alesis verb that would be more worthy of the attention).

jksuperstar · Posted: Tue Dec 14, 2010 11:55 am Post subject:

Improvements to the power supply might have the most bang for the buck. Find out what the supply is providing (12v, 5v, -12v, etc), then look to either replace it, or add a few caps to improve the filtering.

Just a suggestion.

jumunius · Posted: Wed Dec 15, 2010 1:26 am Post subject:

Thanks for the tip. I forget about the power supply, maybe because swapping op amps seems more, er, sexy.

Well, I am not yet sure if I'm going to crack open that DSP128 again, but even if not I have a number of dusty old rack pieces that might be helped by your advice.

jumunius · Posted: Tue Jan 11, 2011 12:58 am Post subject:

Hi, so I almost junked this thing and then decided that something cheap and expendable might be worth fixing up as a learning experience. So, indeed simply replacing the filtering caps stabilized it. However, even after replacing 1/2 the electrolytics on the board (including all around power supply) it's signal is still very unpleasantly noisy. I compared it to a Quadraverb and the Quadraverb is whisper quiet by comparison.

So from here, what part of the power supply should be replaced? The transformer box? In terms of what the unit uses, I see 3 voltage regulators, but they are sheathed in big heatsinks and I can't easily read or poke around.

Thanks in advance for any advice.

jumunius · Posted: Tue Jan 11, 2011 1:05 am Post subject:

As an addendum, the noise is there when the unit is turned off too. I'd think it was something in my setup, except when I tested the Quadraverb using all the same connections I got no noise.

crochambeau · Joined: May 18, 2008 Posts: 31 Location: earth.

It adds noise while powered off?

Sounds like a ground loop to me, while you have it open can you confirm that the mains wiring and ground is in good condition?

I have a couple DSP-128+ units. They do have a somewhat dingy character compared to other multi-effects from that era (this I find favorable). I like the fact that they respond to real time MIDI control for all parameters, something I've always been impressed with given the age of the unit.

Maybe I should join you in tinkering with these dinosaurs.

jumunius · Posted: Tue Jan 11, 2011 6:07 pm Post subject: